The present invention relates to a circuit arrangement for sensing the current flowing in an output transistor to enable the output transistor to be protected against a short circuit.
The need to protect output transistors from short circuits on their load side is well known and there have been a number of different proposals for monitoring the current flowing in an output transistor in order to provide short circuit protection. Most of these prior proposals have required the insertion of a sensing element in series with the output transistor but this solution is not efficient and can result in the need for substantial heat dissipation. Alternative active techniques have been used to shunt current away from the base of an output transistor to limit output current in the event of a large transient short circuit current.
For extended electrical overload resulting from a prolonged short circuit condition at the output, thermal sensing has been used. A disadvantage of the thermal shut-down short circuit protection is the slow response time.
A further active technique is disclosed in U.S. Pat. No. 5,256,914 where the output of an integrated circuit device is monitored in order to detect occurrence of a short circuit condition. This is accomplished by sensing and comparing the respective states of signals at the input and output of the integrated circuit device. The occurrence of an out of state condition between the input and output indicates a short circuit condition.